1. Field of the Invention
The present invention relates to radio base stations and receiver fault diagnosis methods, and more particularly, to a radio base station having a transmitter for a diagnostic test and a receiver fault diagnosis method for the radio base station.
2. Description of the Related Art
Recent mobile communication systems are used not only for voice calls but also widely for data communication. As the number of data communication users has been increased, it is demanded that the data communication rate be increased. To handle the increased number of users and the demand for a higher data communication rate, a recent base station has started using multiple carriers. The number of carriers the base station transmits and receives at the same time has been increased and carrier allocation has become denser.
For the third generation mobile communication systems, new mobile communication methods are currently being standardized, such as a long term evolution (LTE) method and a ultra mobile broadband (UMB) method, which use orthogonal frequency division multiple access (OFDMA), having a higher frequency use efficiency than code division multiple access (CDMA), which has been used. In the future, it is expected that the conventional CDMA and OFDMA will be both used in an identical base station at an identical frequency band. Therefore, it is anticipated that the number of carriers will be increased and the carrier allocation will become denser, even more in the future.
FIG. 1A and FIG. 1B show example electromagnetic wave environments at a multi-carrier base station.
In the base station, an intermodulation (IM) component, which is an intermodulation distortion component, is generated by an amplifier or other devices, for example. As shown in FIG. 1A, for example, two desired wave carriers 101 produce third-order intermodulation distortion (IM3) components 102. When a signal having two frequencies f1 and f2 is input to an amplifier or other devices, their second harmonics are produced by the non-linearity of the device. These second harmonics and the fundamentals generate frequency components (2f1-f2) and (2f2-f1), which are generally called IM3.
As described above, as the number of carriers is increased and the carrier allocation becomes denser in the base station, produced IM3 components 102 are also increased as shown in FIG. 1B. Since the IM3 components 102 overlap with the desired wave carriers 101, the IM3 components 102 serve as noise, affecting the system largely in some cases. Therefore, if the intermodulation distortion characteristics of the base station deteriorate, it influences the system largely in some cases.
During operation, system stability is important, and if a fault occurs, it is demanded that the fault be detected quickly and the original condition be restored. Faults to be detected include, for example, not only a communication disconnection but also a lowered throughput in communication. In the latter case, an external environment may affect, which is peculiar to radio units. To determine the cause of the fault, fault diagnosis is an indispensable function for the radio base station.
It is relatively easy to implement fault diagnosis in a transmitter by extracting a part of a main transmission signal and monitoring it. However, it is difficult to detect a fault in a receiver with the same method because the electric power of a signal input to the receiver varies moment by moment. Therefore, various techniques have been proposed so far to detect a fault in a receiver.
As a first example technique, Japanese Unexamined Patent Application Publication No. Hei-11-154903 discloses the determination of a defective portion in a receiver of a base station by means of electric power. As a second example technique, Japanese Unexamined Patent Application Publication No. 2001-127715 discloses the determination of a defect by using a part of the output signal of a local oscillator of a receiver as a test signal and measuring the electric power of the test signal received. As a third example technique, Japanese Unexamined Patent Application Publication No. 2005-151189 discloses fault diagnosis in which a base station has a terminal function section to allow tests for radio characteristics such as receiving sensitivity to be performed during operation.
These techniques mainly detect a gain error as a receiver fault. The detection of an error in the intermodulation distortion characteristics of a base station has not been disclosed.